Baobabs undergoing localised extinction

The stately baobab is an iconic African tree revered by communities and utilised by wildlife. In recent years, nine of the 13 oldest baobabs in its southern range have died or lost huge chunks, leading experts to believe the species might be undergoing localised extinction.

“Our research pointed out that the largest and oldest baobabs have died at an alarming rate, and because it is happening over a broad region, we needed to look for a mechanism that operates at that scale, like climate,” explains Dr Stephan Woodborne from iThemba Laboratories.

“Our hypothesis is that as temperatures increase in Southern Africa, the trees at the margin of the distribution are most adversely affected, and slowly we might see the shift in the distribution,” explains Woodborne.

It is not just the older trees that are dying off rapidly, younger baobabs are dying too. Adding to the concern, baobabs are not showing healthy recruitment of seedlings in the Kruger National Park, meaning propagation is not taking place.

“In the course of our research we were able to conduct radiocarbon dating on four baobabs that had fallen over,” says Woodborne. “Most of the very large baobabs that are seen in Southern Africa are typically in the age range of 1 000 years.” The oldest baobab recorded is 2 500 years of age.

Radiocarbon dating of old baobabs has revealed these massive trees have weathered severe droughts before, so why are they dying off now? Baobab wood holds a 1 000-year record of rainfall variability across Southern Africa, even revealing evidence that the Little Ice Age also manifested in this region. The climate during the Little Ice Age was a lot cooler and very dry, and because it was cooler, baobabs were able to withstand the drought.

“At present Southern Africa is warming faster than almost any other region on earth,” explains Woodborne. The temperatures that the 1 000-year old baobabs are enduring now are unprecedented.” Baobabs are experiencing hot and dry conditions for the first time and are unable to cope.

The death of these baobabs does not mean all of the trees in the southern distribution won’t be able to adapt. It depends on the microclimate around each tree. Factors like topography, drainage lines, wind, and soil quality influence how each tree is able to adapt to its changing environment. Baobabs do not like frost or winter rainfall.

Baobabs are not only dying in continental Africa but in Madagascar too. Madagascar is home to six of the nine species of baobabs. Exactly how many Madagascan baobabs have fallen over has not been quantified.

However, when the island’s older trees were tested, “there was an omnipresent mush inside them that indicates a kind of rot in their core”, explains Woodborne. This was a common factor among all the baobabs that toppled over in continental Africa.

Baobabs occur naturally along the Limpopo, Zambezi and Kunene rivers and their surrounds.
Scientists speculate there are trees outside this natural distribution as they are relatively easy to propagate. Large baobabs are often found in important archaeological sites. “When looking closely at baobab distribution, which is a bit patchy, it is likely their past distribution was more connected, but it has slowly been shrinking.

“Due to climate change this process may have sped up,” says Woodborne. While the baobab might not be a feature of the Kruger and other protected areas in the future, it is important to clarify that this mighty tree is not going extinct. Populations in its northern distribution remain healthy.

Written by Georgina Lockwood

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